With the development of electronic and information communication industries, individuals carry various kinds of personal digital assistants, office devices, and the like. Therefore, miniaturization of devices has been rapidly advanced in many fields including cellular phones, portable AV devices, portable OA devices, and the like.
However, the size of power sources is not decreased in proportion to the miniaturization and portability of electronic devices. Accordingly, there is an urgent need for lithium secondary batteries which have a compact structure while securing excellent performance with increased energy density.
On the other hand, existing lithium secondary batteries are basically composed of an active material, separators, a liquid electrolyte and a carbonous negative electrode. Existing lithium secondary batteries have restriction in miniaturization due to a complex structure thereof. In addition, it is difficult to fabricate a thin lithium secondary battery due to the use of a pouch, and there is a risk of explosion.
In order to overcome such problems of existing lithium secondary batteries, a thin film battery may include a first thin film, an electrolyte, a second thin film, and the like.
The thin film battery is formed by sequentially forming all of these solid components as films. In this case, the thin film battery may be fabricated to a thickness of a few tens of micrometers, thereby enabling miniaturization of the thin film battery. Also, since the thin film battery has no risk of explosion as compared with existing lithium secondary batteries, the thin film battery is stable and can be implemented as batteries having various patterns according to the shape of a mask.
In order to protect the first thin film, the electrolyte and the second thin film from the external environment, the thin film battery is covered with a protective film made of a material such as polymer, ceramic or metal. However, when the thin film battery is used under a general atmospheric environment or high-temperature atmosphere, micro-pinholes can be produced in the protective film during the formation of the protective film, or curing or deformation can occur in the protective film of a polymer material due to high temperature. Therefore, some components constituting the thin film battery can be exposed to the external environment due to deficiency of the protective film. When some components of the thin film battery are exposed to the external environment, durability and charge/discharge efficiency of the thin film battery can be significantly reduced.
Moreover, a conventional thin film battery composed of unit cells has a limited applicability as a power source for electronic devices due to low capacity.